A cellulose acylate film has high transparency and a mechanical strength, and favorable dimensional stability with minimal dependency on humidity and temperature. Accordingly, this film is broadly used as a support for optical materials requiring such characteristics as described above. In general, the cellulose acylate film is manufactured by casting a dope containing a polymer dissolved in a solvent, over a non-discontinuous (that is, ‘continuous’) support.
A casting method may be classified into two types that include a belt casting and a drum casting in respect of the kinds of continuous supports. The belt casting is a method wherein a dope is casted on a belt, a some amount of solvent is dried and stripped off (detached) and a film is transported to a post-stage process, while the drum casting is a method wherein a dope is casted over a drum and stripped off with substantially little drying, and a film is transported to a post-stage process.
The belt casting generally controls dry conditions and is useful for manufacturing a variety of films, while the drum casting may attain high speed casting, thus enabling mass production. Both the foregoing methods have a disadvantage in that the continuous support, that is, the belt and the drum must be minutely surface-processed to produce a film having the clean surface required for a liquid display.
In order to minutely process the surface of the continuous support, both the belt and the drum must undergo surface abrasion with high precision. For the belt, a belt made of stainless steel is fabricated through grinding and abrasion. The drum is fabricated by nickel plating and hard chromium plating a carbon steel drum then abrasion thereof.
Although the drum does not involve problems, the belt must have a joint 11 formed to link both ends of the belt, thus forming a continuous support as shown in FIG. 1. Joint formation may be accomplished through welding and, in general, the welding method includes tig welding, laser welding, etc. The welding may be conducted before or after abrasion. Since a welded part may remain on the belt after welding, the welded part should be clearly abraded even without encountering problems during film formation. However, the welded part cannot be completely removed, instead, is visibly distinguishable. Accordingly, when a polarizer or an optical compensation film is manufactured using the prepared cellulose acylate, some products may be used after cutting out a welding line therefrom.
In recent years, as a liquid crystal display tends to be increased in a size, widths of a polarizer and an optical compensation film are increased in response to the increase in size described above. Accordingly, in order to the tendency of an increase in width and to allow increase of product yield in manufacturing polarizer and optical compensation films, there are needs for development of an improved cellulose acylate film and for widening thereof.
The drum may be fabricated into a drum having a width of 2030 mm or more, in turn enabling production of a wide film. On the other hand, it is known that the belt production is limited to realize a belt having a maximum width of 2030 mm. Therefore, referring to FIG. 2, in order to fabricate a belt having a width of 2030 mm or more, two different belts may be welded in a length direction. Meanwhile, since a central joint Cc formed through welding between a first belt 10 and a second belt 20, the prepared film has a mark caused by the central joint Cc, in turn encountering difficulty in the use thereof.
The casted dope on the belt is stripped off and formed into a film by a tenter and a drier. The film is slightly shrunk in a width direction after casting to a stripping operation, and optionally subjected to stretching in the tenter. In a drying process using the drier, the film does not show high numerical variation. In order to smoothly transport a normal film and maintain overall characteristics of the film, a trimming process of cutting both ends of the film is adopted wherein the casting process is conducted at one or two sites after casting, tenter processing, drying, etc.
Accordingly, considering a belt with a width of 2030 mm, a width of a film to be manufactured is 2.03 m or less. Further, in consideration of stability in practical casting, stretching at the tenter and removal of both ends of the film, a maximum width possibly obtained may be 1800 mm or less, thus entailing difficulty in manufacturing a film to be in accordance with the current widening trend.